Require Meson 0.48 to get rid of warnings

Debian buster (stable) provides Meson 0.49, which is also available in
stretch (oldstable) backports. It's time to abandon Meson 0.37.

Ref: 20b3f101a4

meson.build

@@ -1,6 +1,6 @@
 project('scrcpy', 'c',
         version: '1.12.1',
-        meson_version: '>= 0.37',
+        meson_version: '>= 0.48',
         default_options: [
             'c_std=c11',
             'warning_level=2',

server/meson.build

@@ -3,7 +3,9 @@
 prebuilt_server = get_option('prebuilt_server')
 if prebuilt_server == ''
     custom_target('scrcpy-server',
-                  build_always: true,  # gradle is responsible for tracking source changes
+                  # gradle is responsible for tracking source changes
+                  build_by_default: true,
+                  build_always_stale: true,
                   output: 'scrcpy-server',
                   command: [find_program('./scripts/build-wrapper.sh'), meson.current_source_dir(), '@OUTPUT@', get_option('buildtype')],
                   console: true,